Foam preparations of sclerosant chemicals can be very useful in the treatment of varicose veins and other malformed vessels in the body. The injection of foam into varicose veins has many advantages over the injection of sclerosant liquid; a foamed product allows a large area of internal vessel wall to be exposed to the active, for a given total quantity of the active in the vein. Foam with the right characteristics is able to displace blood from the vein, preventing dilution of the drug, as happens with liquid.
It is likely that many other medical applications for clinical foam preparations will be developed in the future, including but not limited to the sclerosing of the internal surfaces of other vessels and cavities in the body.
A problem with foamed clinical preparations is the production of a consistent, sterile product for use by a physician. A further issue is the potential danger of injecting quantities of air into the body, since nitrogen is highly insoluble. Extemporaneously prepared foams made with air are widely used, but work conducted by the inventors has shown that even the smallest volumes of nitrogen, of the order of a few percent, give rise to bubbles which endure in the vascular system and are potentially harmful. These issues are discussed at length in co-pending applications numbers PCT/GB04/004824, PCT/GB04/004831, PCT/GB04/004846 and PCT/GB04/004848, the contents of which are incorporated herein by reference.
Many of the current extemporaneous methods of foam production rely on drawing a small quantity of liquid into a syringe, together with a large quantity of air, and then agitating the mixture by some means to produce a foam. It is possible using these techniques to produce a good quality homogeneous foam with microscopic bubbles (“microfoam”), but it is equally possible to produce a poor quality foam: the properties of the foam are dependent on a number of factors including the amount of energy put into making the foam, and the ratio of gas to liquid in the syringe.
It is preferable to prepare the foam with gasses such as oxygen or carbon dioxide, which will be absorbed by the body much more readily; this is discussed in other patents and patent applications in the name of the applicants details of which are given above. However, there are difficulties for a physician wishing to fill a syringe exclusively with a gas other than air. The most straightforward method would be to connect the nozzle of the syringe to a line from a cylinder of the desired gas, but it is difficult reliably to exclude all air/nitrogen and virtually impossible to check the purity of the gas component in the syringe. Alternatively, one could fill a fume cupboard or similar with an appropriate gas or gas mixture and fill the syringe in the fume cupboard. However, this involves a large amount of gas being provided to fill the fume cupboard when only a very small amount of gas is necessary to fill the syringe. Furthermore, it is still difficult to be confident that the contents of the fume cupboard, and therefore the gas in the filled syringe, are exclusively the desired gas or gas mix.